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Listen to part of a lecture in a biology class. The professor is discussing the migration patterns of monarch butterflies. Every year, millions of monarch butterflies travel up to 3,000 miles from the United States and Canada to central Mexico. What's remarkable is that no single butterfly makes the entire round trip. Instead, it takes three to four generations to complete the journey south, and then a single generation flies back north in the spring. How do they know where to go? Recent research suggests that monarchs use a combination of the sun's position and an internal magnetic compass. However, the sun's position changes with time of day, so butterflies must compensate using their internal circadian clock. Interestingly, if their clock is disrupted—for example, by exposure to constant light—they lose their ability to navigate. This shows the crucial link between their biological clock and environmental cues. Scientists are also concerned that habitat loss and climate change are disrupting these migratory patterns, making conservation efforts increasingly important.